linux-kernel - Re: [PATCH v7 5/8] loop: be paranoid on exit and prevent new additions / removals

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Message-ID: <7e76d892-b5fd-18ec-c96e-cf4537379eba@acm.org>
Date:   Sat, 20 Jun 2020 10:11:46 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Luis Chamberlain <mcgrof@...nel.org>, axboe@...nel.dk,
        viro@...iv.linux.org.uk, gregkh@...uxfoundation.org,
        rostedt@...dmis.org, mingo@...hat.com, jack@...e.cz,
        ming.lei@...hat.com, nstange@...e.de, akpm@...ux-foundation.org
Cc:     mhocko@...e.com, yukuai3@...wei.com, martin.petersen@...cle.com,
        jejb@...ux.ibm.com, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v7 5/8] loop: be paranoid on exit and prevent new
 additions / removals

On 2020-06-19 13:47, Luis Chamberlain wrote:
> Be pedantic on removal as well and hold the mutex.
> This should prevent uses of addition while we exit.
> 
> Reviewed-by: Ming Lei <ming.lei@...hat.com>
> Reviewed-by: Christoph Hellwig <hch@....de>
> Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>
> ---
>  drivers/block/loop.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index c33bbbfd1bd9..d55e1b52f076 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -2402,6 +2402,8 @@ static void __exit loop_exit(void)
>  
>  	range = max_loop ? max_loop << part_shift : 1UL << MINORBITS;
>  
> +	mutex_lock(&loop_ctl_mutex);
> +
>  	idr_for_each(&loop_index_idr, &loop_exit_cb, NULL);
>  	idr_destroy(&loop_index_idr);
>  
> @@ -2409,6 +2411,8 @@ static void __exit loop_exit(void)
>  	unregister_blkdev(LOOP_MAJOR, "loop");
>  
>  	misc_deregister(&loop_misc);
> +
> +	mutex_unlock(&loop_ctl_mutex);
>  }
>  
>  module_init(loop_init);

Is try_module_get(fops->owner) called before a loop device is opened and
is module_put(fops->owner) called after a loop device is closed? Does
that mean that it is impossible to unload the loop driver while a loop
device is open? Does that mean that the above patch is not necessary or
did I perhaps miss something?

Thanks,

Bart.